Thermal process for in situ decomposition of oil shale



United States Patent O T 3,139,928 THERMAL PROCESS FOR IN SITU DECOR [POSI- TION OF OIL SHALE Douglas E. Broussard, Bellaire, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed May 24, 1960, Ser. No. 31,382 5 Claims. (Cl. 166-2) This invention relates to a process for producing hydrocarbons from an impermeable oil-containing earth formation by treating such formations and the materials contained therein in their natural position. More particularly, the present invention is concerned with a process for the in situ retorting of oil shale to recover shale oil.

Oil shale is a rock consisting of limestones, shale and hydrocarbons. The hydrocarbons occur as kerogen and often make up as much as 30 percent of the virgin ore. When heated in a non-oxidizing environment, shale oil gives off hydrocarbon vapors, leaving a porous, permeable and friable rock formation in place.

Oil shale is abundant in the western states of the United States and is found in massive deposits covering many square miles and in beds ranging up to several thousand feet in thickness. These beds however are not of uniform richness and relatively few of the rich veins outcrop at the surface. Although vast reserves of oil in the form of shale oil are potentially available both in this country and abroad, the high costs of mining the oil shale, crushing and grinding it as a preliminary step to any feasible recovery method, such as by high temperature retorting, make shale oil as a source of additional oil completely unfeasible from an economic standpoint. Thus, before shale oil is available in crude form comparable to ordinary crude petroleum, the operations of stripping or drilling the overburden, mining the rock, transporting the rock to the surface and/or a suitable processing point, crushing the shale and grinding the crushed rock to a particle size permitting effective heat treatment and retorting the shale particles at high temperature must be performed. Even though potential recovery of shale oil may approximate as high as 50 to 75 gallons per ton of shale processed, the aggregate mining and processing costs impose such an economic burden that it would seem that shale oil could not become a suflicient energy supply factor unless a real and substantial shortage of petroleum developed.

An object of the present invention is to provide a method for recovering shale oil by in situ means so as to eliminate the costly operations of mining, transporting and processing the rock.

Another object of the present invention is to provide an improved in situ process for the retorting of shale in which the maximum recovery of oil per unit quantity of shale is obtained and the oil recovered is in suitable form for further processing.

A still further object of the present invention is to provide a thermally efficient underground oil shale retorting process which provides a stable and controllable means of retorting at optimum temperatures with maximum recovery of oil from the shale.

Still another object of the present invention is to provide a thermal process for in situ decomposition of oil shale which may be carried out in a shale formation without the necessity of cracking or otherwise shattering the shale formation with auxiliary means.

These and other objects of this invention will be understood from the following description when taken with reference to the attached drawing, wherein a schematic View of a shale oil deposit, taken in crosssection, is shown together with the necessary equipment for practicing the method of the present invention.

3,139,928 Patented July 7., 1964 Virgin oil shale is very strong and impermeable. Therefore, at considerable depths, a cavern can be developed in the virgin ore which can withstand considerable fluid pressure without appreciable leakage of the pressurizing fluid. Shale formations are adapted to withstand fluid pressures approaching the fracturing pressure of normal earth formations. The characteristics of shale formations and their environment are particularly suited for in situ cyclic pressure-temperature processing of the rich oil veins.

According to the present invention, the subject process comprises the steps of drilling one and preferably at least two wells into a shale oil formation for the purpose of recovering hydrocarbons therefrom. Communication is established between the shale oil-bearing formation and the ground surface through two conduits in preferably at least two wells. Caverns are then formed near the bottom of each of the wells in the shale oil-bearing formation. The wells are spaced at distances so that as the process progresses in each of the caverns there is always a portion of the shale bearing formation between the two caverns so that at no time do the caverns interconnect.

At the beginning of a process no caverns exist at the bottom of the wells but these caverns are formed by carrying out the method of the present invention. In starting up the present process in a shale oil-bearing formation, the surface temperature of the walls of the borehole in the shale oil formation is raised to an elevated temperature which, when exposed to an oxidizing atmosphere, 'promotes partial spontaneous combustion of the exposed formation, say, to a temperature of between 500 and 900 F. The step of originally raising the temperature of the surrounding rock may be carried out in any manner well known to the art, such as by use of an electrical heater positioned adjacent the formation or by burning a downhole flame or combustible gases. Application of heat in this manner cracks the kerogen in the exposed rocks and releases hydrocarbon vapors. A limited quantity of oxygen is then injected into the well or the caverns at the bottom thereof for further heat generation through combustion of some of the hydrocarbons. Oxygen may be supplied in the form of air, or oxygen mixed with other gases, or slugs of pure oxygen may be intemittently injected down the well.

Simultaneously, or shortly after injecting air into the underground cavern, a fluid, preferably another gas, is injected into the cavern through the well to increase the pressure within the cavern, thereby compressing the downhole hot gases and conveying them to the extremities of the cavern or borehole. The injected fluid can be either a gas, a liquid, or a mixture of the two. Preferably, air and the gas injected to increase the pressure in the cavern are injected through separate supply tubes within the well borehole. The cavern is then maintained at these conditions of temperature and pressure for a time sufficient for heat to pass into previously unheated portions of the shale oil-bearing formation adjacent the cavern. Subsequently, the pressure within the cavern is released allowing the gases therein to escape to the surface where the hydrocarbon-rich gases are compressed, condensed and separated from hydrocarbon fluids. The dry gas, which is without free oxygen, is then stored or pumped into a second well with the desired quantity of free oxygen to pressure-up a second cavern at the bottom thereof in the manner previously described hereinabove. As pressure is released from a cavern and hydrocarbon-rich gas comes to the top for treatment, additional gas is preferably pumped out of the well or cavern to be treated.

As the hydrocarbon-rich gas escapes to the surface, the porosity and permeability of the previously heated shale oil formation is increased. At this point the cycle starts all over again with the cavern being repressurized by supplying the desired quantity of air for further heat generation within the cavern followed by gas injection to build 'up the pressure therein.

A typical installation for carrying out the process of the present invention is shown in the drawing wherein a pair of wells 11 and 12 extend from the surface of the earth 13 down through various formations 14 until they penetrate a shale oil-bearing formation 15. The wells 11 and 12 are preferably cased in a normal manner by well casings 17 and 18 which are closed at the tops by any suitable type of closure means 19 and 20, and, if desired at the bottom by closures or packers 19a and 20a. Extending down through the cased wells are air or oxygen supply tubes 21 and 22 provided with flow control valves 23 and 24 which may be either manually or power actuated. The air injection tubes 21 and 22 are provided with perforated sections 25 and 26, respectively, at their lower ends for injecting gas into the caverns 27 and 28 which are formed at the bottom of the wells after the method of the present invention has progressed for some time.

The well casings 17 and 18 are each provided with a second string of pipe 31 and 32, respectively, which are preferably of larger diameter than the air supply tubes 21 and 22. The larger pipe strings 31 and 32 within the well casings serve as a means for supplying gas to the bottom of the borehole under pressure, and also serve as producing tubes for allowing hydrocarbon-rich gas from the caverns 27 and 28 to escape and/ or to be pumped out of the wells. The lower ends of the pipe strings 31 and 32 are open or are provided with perforate sections 33 and 34, as illustrated.

At the surface an air compressor 35, air storage tank 36 and air heater 37 are provided for supplying heated air, or an oxygen-containing gas, through conduits 38, 39 and 40 to the air supply tubes 21 and 22 of the two wells. Instead of employing separate air tubes 21 and 22 which extend all the Way down to the bottom of the well, the annular spaces 41 and 42 outside the pipe strings 31 and 32 and within the well casings 17 and 18 could be employed as suitable conduits for conducting air to the bottom of the well.

The gas supply and producing tubes for pipes 31 and 32 are in communication through pipelines 43 and 44 with a valve manifold system 45. Thus, when hydrocarbonrich gas is released from the producing tube 31 by opening valve 46 with valves 47, 48 and 49 being closed, the gas is directed through compressor 50 which forces it through condenser 51 into a gas storage and separator 52. The separator tank 52 is provided with liquid-level control means such as a float 53 for operating a valve 54 in the liquid discharge line 55 to a liquid storage tank 56. A portion of the gas from the top of the separator tank 52 may be directed through pipeline 57 to be sold, or alternatively may be directed through line 58 back through the valve manifold system 45 where, with valve 47 closed and valve 48 open, it is directed through line 44 and injected through the gas supply tube 32 into the second well at the point in the cycle of operations when the gas is to be injected into the second cavern 28 to increase the downhole pressure, thereby compressing the downhole hot gases and conveying them to the extremities of the borehole or cavern 28.

The present method provides an in situ combustion process for use in the shale oil-bearing formations having little or no permeability in their natural state. At all times while the method of the present invention is being carried out there is an impermeable mass of oil shale between caverns 27 and 28. Thus, it is impossible to use the conventional in situ combustion method wherein a continuous flow of heated gases and liquids would pass through the formation from one well to another.

Since the processed zone is maintained at a high tem perature the pore volume is filled with gases and vapors which can be compressed during the pressurizing phase and expanded during the de-pressurizing phase of the cycle, thereby permitting flow of heated gases into the previously treated zones surrounding the wellbore during pressurization and providing the pressure for reversed flow during the blow-down or producing phase of the cycle which takes place when the pressure within a well is released. The heated processed shale Zone supplies the necessary heat to raise the air and residual fuel mixture to the combustion temperature.

During the carrying out of the method of the present invention, the heat is transferred to the virgin shale ore surrounding the borehole and caverns by both conduction and convection. Transfer by convection is accomplished as an effect of the reversing flow of fluids through the process zone. Furthermore, the present method can be adjusted to expand the process zone at the rate of heat flow through the virgin ore, thereby resulting in very little loss of non-productive thermal energy in the subsurface formations. An important feature of the present method is that the process expands fastest in the direction of the most productive portions of the ore body since the greatest porosity and permeability of a formation is developed in the richest veins of the virgin shale ore.

Preferably, the method of the present invention is employed using two or more wells in order to minimize the surface storage of hot gases and vapors. By operating two or more wells out of phase with each other, the produced gases from one can be used to provide the source of gas for pressurization of the other after the liquid components have been extracted therefrom. The produced gases are passed through suitable apparatus such as a compressor, condenser and separator and capture the desirable hydrocarbons before injecting the dry gases into a second well. The present process is also applicable to certain tar sands in areas where there is little permeability to the formation and sufiicient overburden exists to withstand the required pressures at the bottom of the well.

I claim as my invention:

1. A method for processing in situ an impermeable shale oil-bearing formation for the purpose of recovering hydrocarbons therefrom wherein a permeable cavern is formed in said shale formation at a point traversed by a well, said method comprising the steps of establishing communication between the shale oil-bearing formation and the ground surface through a well normally closed at the top, initially raising the temperature of the walls of said closed well to an elevated temperature promoting partial spontaneous combustion of the exposed formation to release the hydrocarbons from the formation exposed to heat, injecting a quantity of free oxygen-containing gas into said closed well to promote heat generation therein, increasing the pressure within said cavern by injecting another gas which is substantially oxygen free and hydrocarbon free into said closed well to compress the hot gases in said well, maintaining said closed well at said conditions for a time sufficient for heat to pass into previously unheated portions of said shale oil-bearing formation adjacent said well, and subsequently releasing the pressure from said well to produce hydrocarbonrieh heated gas from said well.

2. A method for processing in situ an impermeable shale oil-bearing formation for the purpose of recovering hydrocarbons therefrom wherein permeable caverns are formed in said shale formation at points traversed by wells, said method comprising the steps of establishing communication between the shale oil-bearing formation and the ground surface through a well normally closed at the top, forming at least one cavern in said oil-bearing formation traversed by said closed well by initially raising the temperature of the shale formation forming the walls of said well to an elevated temperature promoting partial spontaneous combustion of the exposed formation to release the hydrocarbons from the formation exposed to heat, injecting a quantity of free oxygen-containing gas into said cavern of said closed well to promote heat generation therein, increasing the pressure within said cavern by injecting another gas which is substantially oxygen free and hydrocarbon free into 'said cavern of said closed well to compress the hot gases in said cavern, maintaining said cavern of said closed well at said conditions for a time sufficient for heat to pass into previously unheated portions of said shale oil-bearing formation adjacent said cavern, subsequently releasing the pressure from said first cavern by opening said closed well to produce hydrocarbon-rich heated gas from said cavern via said well, separating the major portion of said hydrocarbons from said gas, and retaining a quantity of the substantially liquidfree and oxygen-free separated gas for re-injection into the heated cavern after another injection of oxygen-containing gas has been forced into the cavern on a subsequent cycle.

3. A method for processing in situ an impermeable shale oil-bearing formation penetrated by at least two wells for the purpose of recovering hydrocarbons therefrom, wherein permeable caverns are formed in said shale formation at points traversed by wells, said method comprising the steps of establishing communication between the shale oil-bearing formation and the ground surface through at least two conduits in first and second wells, forming first and second caverns in said oil-bearing formation in said first and second wells respectively, therebeing a portion of impermeable shale-bearing formation between said caverns at all times, said first cavern being formed by initially closing said first well, raising the temperature of the shale-formation forming the walls of said first well to an elevated temperature promoting partial spontaneous combustion of the exposed formation to release the hydrocarbons from the formation exposed to heat, injecting a quantity of oxygen-containing gas into said first cavern of said closed first well to facilitate the heat generation therein, increasing the pressure within said first cavern by injecting another gas which is substantially oxygen free and hydrocarbon free into said first cavern to compress the hot gases in said first cavern, maintaining said first cavern of said closed first well at said conditions for a time sufiicient for heat to pass into previously unheated portions of said shale-bearing formation adjacent said first cavern, and subsequently releasing the pressure from said first cavern by opening said closed first well to produce hydrocarbon-rich heated gas from said first cavern via said first well.

4. A method for processing in situ an impermeable shale oil-bearing formation penetrated by at least two wells for the purpose of recovering hydrocarbons therefrom, wherein permeable caverns are formed in said shale formation at points traversed by wells, said method comprising the steps of establishing communication between the shale oil-bearing formation and the ground surface through at least two conduits in first and second wells, forming first and second caverns in said oil-bearing formation in said first and second wells respectively, therebeing a portion of impermeable shale-bearing formation between said caverns at all times, said caverns being formed by initially closing said first well, raising the temperature of the shale formation forming the walls of said first well to an elevated temperature promoting partial spontaneous combustion of the exposed formation to release the hydrocarbons from the formation exposed to heat, injecting a quantity of air into said first cavern of said closed first well to facilitate the heat generation therein, simultaneously increasing the pressure Within said cavern of said closed first well by injecting a gas which is substantially oxygen free and hydrocarbon free supplied from said second well into said first cavern to compress the hot gases in said first cavern, maintaining said first cavern of said closed first well at said conditions for a time sufiicient for heat to pass into previously unheated portions of said shale-bearing formation adjacent said first cavern, subsequently releasing the pressure from said first cavern by opening said closed first well and pumping hydrocarbon-rich heated gas from said first cavern via said first well, separating the major portion of said hydrocarbons from said gas, pumping a quantity of the substantially hydrocarbon-free and free oxygen separated gas into said heated second well to build up the pressure therein and carry out in said second well the same steps previously carried out in said first cavern in order to form said second cavern.

5. A method for processing in situ an impermeable shale oil-bearing formation penetrated by at least two wells for the purpose of recovering hydrocarbons therefrom, wherein permeable caverns are formed in said shale formation at points traversed by wells, said method comprising the steps of establishing communication between the shale oil-bearing formation and the ground surface through at least two conduits in first and second wells, forming first and second caverns in said oil-bearing formation at the bottom of said first and second wells respectively, therebeing a portion of impermeable shale-bearing formation between said caverns at all times, said caverns being formed by initially closing said first well, raising the temperature of the shale formation forming the walls of said first well to between 500 and 900 F. to release the hydrocarbons from the formation exposed to heat, injecting a quantity of air into said first cavern to facilitate the heat generation therein, subsequently increasing the pressure within said caverns by injecting a gas which is substantially oxygen free and hydrocarbon free supplied from said second well into said first cavern to compress the hot gases in said first cavern, maintaining said first cavern at said conditions for a time sufficient for heat to pass into previously unheated portions of said shalebearing formation adjacent said first cavern, subsequently releasing the pressure from said first cavern by opening said closed first well and pumping hydrocarbon-rich heated gas from said first cavern via said first well, compressing said hydrocarbon-rich gas, condensing the major portion of said condensable hydrocarbons from said gas, separating the major portion of said hydrocarbons from said gas, pumping a quantity of the substantially remaining gas into said heated second well to build up the pressure therein and carry out in said second well the same steps previously carried out in said first cavern in order to form said second cavern.

References Cited in the file of this patent UNITED STATES PATENTS 1,342,741 Day June 8, 1920 1,422,204 Hoover et a1 July 11, 1922 1,457,479 Wolcott June 5, 1923 2,630,306 Evans Mar. 3, 1953 2,853,136 Moore et a1 Sept. 23, 1958 

1. A METHOD FOR PROCESSING IN SITU AN IMPERMEABLE SHALE OIL-BEARING FORMATION FOR THE PURPOSE OF RECOVERING HYDROCARBONS THEREFROM WHEREIN A PERMEABLE CAVERN IS FORMED IN SAID SHALE FORMATION AT A POINT TRAVERSED BY A WELL, SAID METHOD COMPRISING THE STEPS OF ESTABLISHING COMMUNICATION BETWEEN THE SHALE OIL-BEARING FORMATION AND THE GROUND SURFACE THROUGH A WELL NORMALLY CLOSED AT THE TOP, INITIALLY RAISING THE TEMPERATURE OF THE WALLS OF SAID CLOSED WELL TO AN ELEVATED TEMPERATURE PROMOTING PARTIAL SPONTANEOUS COMBUSTION OF THE EXPOSED FORMATION TO RELEASE THE HYDROCARBONS FROM THE FORMATION EXPOSED TO HEAT, INJECTING A QUANTITY OF FREE OXYGEN-CONTAINING GAS INTO SAID CLOSED WELL TO PROMOTE HEAT GENERATION THEREIN, INCREASING THE PRESSURE WTIHIN SAID CAVERN BY INJECTING ANOTHER GAS WHICH IS SUBSTANTIALLY OXYGEN FREE AND HYDROCARBON FREE INTO SAID CLOSED WELL TO COMPRESS AND HOT GASES IN SAID WELL, MAINTAINING SAID CLOSED WELL AT SAID CONDITIONS FOR A TIME SUFFICIENT FOR HEAT TO PASS INTO PREVIOUSLY UNHEATED PORTIONS OF SAID SHALE OIL-BEARING FORMATION ADJACENT SAIOD WELL, AND SUBSEQUENTLY RELEASING THE PRESSURE FROM SAID WELL TO PRODUCE HYDROCARBONRICH HEATED GAS FROM SAID WELL. 